Tohatsu MD90A surging around 800-1000rpm

My Tohatsu surging revs when just engaged in gear. It is slight, perhaps a surge of 100rpm and with a frequency of every 1-2 seconds. At low rpm setting at idle it does not surge, at rpm above about 1300rpm it also does not seem to surge, certainly not at full throttle.

I have done the following:

Air injectors have just been changed and to an updated version of what was there before.
I reset the TPS with the procedure of pulling out the kill switch button a couple of times until it beeps 3 times.
Anything else to try? I will change the air filter into the air compressor and probably also the high pressure fuel filter.

Thanks in advance.

Any codes showing?

pvanv said:

Any codes showing?

Click to expand...

No, not the last time I checked. I will double check again. Today I was in the boat. At the 700rpm idle setting it seems to idle without surging. When in gear from there to around 1300rpm it surges. Above about 1300rpm it runs without surging.

pvanv said:

Any codes showing?

Click to expand...

Hi Paul,

I just double checked. No codes showing. I have some videos of the engine running at idle if it helps but I can describe it as a subtle varying of revs at idle and it appears to be a bit worse when in gear, so much so that it makes the boat speed surge slightly.

I was going to check the air and fuel regulators but don't know how to remove them or what to look for once I have them out. Are they just press fit in like the air injectors and just need to be carefully prised out?

My battery is a Shield Powerstation MF31-110R which is 110Ah and says Amps (SAE) 680 on it. It is about 7 years old but reads 14.6v when the engine is at idle. It is always kept charged with a smart charger.

Any advice on what to check would be appreciated!

Cheers,

Tim

All those 90s had a bit of a lope or surge just above slow idle. Pressures are checked with gauges on the rails.

pvanv said:

All those 90s had a bit of a lope or surge just above slow idle. Pressures are checked with gauges on the rails.

Click to expand...

Hi Paul, thanks for the response. It also wants to stall sometimes at a specific rev just above idle. If I push through it picks up again not problem. This problem has only started after I changed all the air injectors. I checked the air rail with a mountain bike suspension pump with a digital gauge, it was reading 74psi while turning over, but I will check it again while it is running. I don't have a gauge to check the fuel pressure but when I depress the valve after running it, fuel spurts out 10 cm or so which apparently is ok.

Would you say my battery spec is ok? I read a lot about weak batteries causing the problems but I cannot find a definitely answer as to what battery spec I need for the Ah and CCA (further confused by the fact there seem to be several different versions; CCA, SAE, EN, MCA).

Thanks again for your help.

Cheers,

Tim

1000 cca battery. Air pressure about 10 psi above fuel.

Thanks for the info. I have read 1000cca is for cold climates, but still it should be 800.

I today I did the following:

- took out the fuel regulator and it seemed fine. Cleaned the outside and mess filter and reinstalled it.
- took out the air regulator and it was a bit dirty so cleaned it up and reinstalled.
- took out the plugs and they were all completely black and sooty, not sticky. The plugs were not like this before I replaced the air injectors. The air filter looks a bit dark inside so I guess I should replace it but I don't think this would cause the spark plugs to soot up. What would cause this? Would the engine running rich cause this surging in revs?

Cheers,

Tim

Yes, it could.

pvanv said:

Yes, it could.

Click to expand...

It could cause the rev surging? Any suggestions as to what to check to fix the rich running? Assuming at this stage it is not the air filter or an under powered battery issue (although given that the battery can cause weird running behaviour, could this mess with the fuel/air mix by not initialising the ECU correctly?).

Thanks again for your help.

If the battery is too small, the ECU may not initialize properly, and virtually anything could go wrong.

Ok, that for sure seems to be clear from all the reading I have done, including many posts by you. Does the ECU remain "incorrectly initialized" after initial starting and therefore the problems persists beyond the initial starting phase? Doesn't the alternator then take over and provide the appropriate current at some point?

A new battery is not cheap so I want to eliminate as many other possible causes before I invest there, especially checks/cleaning I can do with only time and not money investment. What are the main causes of rich running/dry, sooty plugs?

- Not enough air - blocked air filter? If air rail gives the correct pressure it should be ok?
- Incorrect amount of fuel injected - fuel injector issue or ECU initialization issue? If fuel rail gives correct pressure, is it ok? All fuel/air mix is controlled by ECU?
- Something wrong with the fuel regulator? Any way to test? It seemed clean when I removed it.
- Anything else?

Thanks a lot.

The issue is that the starter motor takes significant amps to crank the motor. This causes a sag in the voltage available to the ECU during cranking, which leads to "god only knows what" issues. This persists while running, just as if a PC that starts incorrectly will have issues until rebooted. You can maybe load test your battery and check specific gravity if it's a flooded type.

Yes, the ECU determines fuel mix.
A gauge set can tell you the fuel and air pressures. They need to be about 10 psi apart (air higher).
Your dealer should be able to help dx the issue.
Again, we have seen that all these 90's have a weirdness just around idle, that is more pronounced in gear, and more pronounced with a solid (SS) prop.

Hi Paul,

Thanks for the details. So today I did not fix the issue, but I did the following checks:

- Checked the air and fuel pressures. My digital gauge reads about 74psi for the air rail and another, analog gauge (sacrificed for the purpose of measuring the fuel pressure but not accurate because the needles wiggles around) was measuring about 10psi higher for the fuel side. You mentioned that air should be higher but my service manual says the opposite so I assume this is ok.
- Removed the air filter while running and it made no difference to the surging. Not sure if this means anything. I guess the filter would need to be really blocked to change anything.
- Measured voltage across the battery while starting and it drops to 11.7v so I guess this is an issue if it needs to be above 12. Trying to source a battery at the moment. Not so easy with the varying CCA standards and also the fact that regulations were brought in not so long ago forcing companies to report more realistic CCA numbers meaning some batteries are now rated lower than they previously were, even though they remain unchanged. Best I have found so far is this: https://www.tayna.co.uk/leisure-batteries/numax/xv31mf/ which is 740 CCA EN or 925 MCA, which just falls short of the 850 CCA SAE requirement (bit hard to convert). This option https://www.varta-automotive.com/en-gb/products/varta-professional-dual-purpose/820-054-080 is 800 CCA EN and 1000 MCA.
- Tried to check the resistance of the water temp and throttle position sensors to check against the values in the service manual, but could not get a good reading. They were only showing something like 2kOhms which doesn't make sense (for the WTS it is supposed to be 2.45ohms at 20 degrees).
- Checked for fault codes again, still nothing. Is there a chance there is something preventing fault codes being logged or preventing the RPM counter reading them?
- I carried out all these checks using fresh fuel purchased today.

Noted your point about these early 90s having some weirdness around idle. I guess I will never get it to hum away quietly but for sure the dirty plugs is not normal.

If you have any other suggestions or advice, I'm all ears. Many thanks again.

Are you running Champion or NGK plugs? The newer plug is the NGK IZFR6Q.
Air pressure should be 80 PSI. Yes, the "air backed" regulator should give fuel at 90 PSI.
If both pressures are low, it's usually an air problem.
If the battery drops below 12 at the battery itself, it will be even lower at the ECU.
The battery should be stable after charging at almost 13v.
MCA is at 32F, CCA is always lower.

Hi Paul,

The plugs are NGK PZFR6H. I have attached an image of them all sooted up here: https://1drv.ms/u/s!AmWxme0O8FrMoGrVi4rwuf6ql6zK

2 avenues of investigation at the moment, battery sourcing and air pressure.

For the battery, as I said, it is hard to find something suitable and stay within my 330mm length requirement. Over here in europe we have CCA EN which is lower than SAE but the conversion is not really defined (I think 800 EN should equate to about 850 SAE). I found that if I go for a car battery (rather than a marine/leisure battery) I can get some higher CCA numbers. The boat has a small fish finder, a manual bilge pump and a switch to light up the gauges that I never use (I don't have nav lights so do not boat at night) so the demands beyond starting are tiny, if not irrelevant I guess. Seems that many people just use car batteries for their boats without too much issue and pay more attention to keeping them topped up over winter. Do you have any comment on this?

For the air pressure, it would be useful to know all the areas to check:

- pipe from compressor to bottom of air rail
- air regulator - is there a way to check that this is working properly?
- how can I verify that the air compressor is producing enough pressure?
- any other likely sources of air leaks?

I had an issue before with the hose between air rail and fuel regulator splitting and then engine would not run. Found the split, cut out that section of hose, put it back on but then found the wire hose clips were not adequate and were allowing air to escape at the hose connections. I have since replaced them with better clips and can no longer feel any air escaping. I have ordered a new hose. All the checks and readings have been taken after I fixed this hose issue. Does this hose along with the fuel regulator only affect the fuel pressure?

Also, I mentioned I tried to measure the resistance across the pins of the water temp sensor and could only get 2000ohms. My service manual says it should be 2.45ohms at 20 degrees C. I tried multiple times and with 2 different volt metres and could not get a good reading. Is this sensor likely to be suspect or am I measuring the wrong thing? Of course, if the sensor is faulty, its not telling the ECU that the engine is warming up. Also couldn't get a good reading from the TPS but could from the fuel injector.

Thanks a lot.

Those plugs look a bit rich.
Car batteries are OK for cranking.
The fuel regulator is supposed to bump up 10 psi from the air pressure.
The higher pressure hoses typically use oitker clamps.
Yes, a bad sensor could tell the ECU to stay richer.
TPS should be about 5 ohms top to bottom. Middle to bottom should vary from about 1/2 ohm closed to about 5 ohms open.
Water sensor about 2.6K at 20C, 0.3K at 80C.
You can use shop air to test if you think the compressor is weak.

Hi Paul,

Just measured the resistance across the the WTS again and I could get a consistent 2.45kOhms (so 2450 Ohms). My service manual says it should be 2.45 Ohms (not 2450 Ohms). My values line up with your values so is my manual wrong?! Seems odd I have exactly the value the manual states but several orders of magnitude different! For the record, I have this manual:

https://www.maxrules.com/TOHCatalogs/servman/003-21052-1.php

Also measured the voltage drop from battery to engine terminals at rest (i.e. no engine running) and it was 0.2v, so during cranking, it was around 11.5v which is still a way above the 10v threshold I believe triggers a low voltage fault but if the ECU needs above 12v during cranking then a fresh battery would certainly not be a bad thing. If I go for a car battery, I could pick one up tonight.

When an engine starts, I guess it is running rich normally (i.e. choked to helped it start) but then leans out as it warms up. Would that mean that in the first few minutes, the plugs are likely to soot up even if the engine was working perfectly and then the soot burns off? I'm just thinking ahead to when I want to see if the engine is running normally again...how long should I run it for before checking the condition of the plugs again?

Thanks again!

Sounds like a typo in the manual. Mine is newer.
A low batt fault would get recorded, but even above that during cranking, weirdness can happen.
The mixture needs to be richer when cold as fuel does not atomize as well when cold. Unless way too rich, it shouldn't soot up the plugs.

I have just taken off my air rail completely and taken out all the parts. The rail itself seems fairly clear but the ends of the fuel injectors are very dirty with caked on dirt. I will try and clean them, but it does not just wipe off. I guess they are working because all the plugs are sooting up suggesting fuel is getting to every cylinder. My air regular also has liquid (I assume petrol) in it. I can see it moving around behind the small mesh filter. Some questions:

1. What is the best solvent to clean the fuel injectors that is safe for the o-rings etc?
2. Is there a method of testing the operation of the fuel injectors by putting them across a battery like you can do with the air injectors?
3. Should there be petrol in my air regulator? Is it possible to test the function and/or clean the air regulator? What actually is it? A spring backed valve that opens to vent to the atmosphere to maintain the rail pressure at 80psi?

Many thanks for your help.

You can send out the injectors to have them cleaned.
I am not aware of a good in the field test for the injectors.
Yes, the air regulator vents to atmosphere. There really should not be fuel on the air side. The fuel regulator which runs in conjunction with the air pressure may have an issue.

Hi Paul,

Yes, I have just discovered that you can get fuel injectors cleaned. I may well do this before next season. I cleaned them up as best I could (luckily the nozzles are fine and it is just the surrounding metal around the o-ring which is corroded) and confirmed that they click when put across a 12v battery. I made sure the air rail was clear and re-assembled everything, got my new battery (100Ah and 830 CCA EN which should be comfortably over 850 CCA SAE), cleaned the plugs and fired it up. Seemed a bit more stable but after running it for 10-15 minutes on the muffs (idle and max 2000rpm for short periods), the plugs still came out sooty (although a bit less sooty than running it for less than 5 minutes previously).

The one benefit of having this issue is that I am learning a lot about the engine. I'll write it down for confirmation but also if it is useful for people referring to this thread int he future...

With regards to fuel being on the air side. When I had the air rail apart, I could see that between the end of the fuel injector and the small metal cone thing with slots in it that fits over the end of the injector, there are gaps, it is not sealed. As far as I could tell, this is an open path into the air side of the air rail? However, are these gaps meant to only suck in air and the jet of fuel from the fuel injector should only fire straight into the right angle channel which then directs it to the air injector? Where exactly does the air mix with the fuel, through the gaps in the slotted cone or in the air injector?

I have also been thinking more about the fuel regulator and after your comments above, I started to try and figure out how it works. The small pipe coming off it (which in the car world seems to be referred to as the vacuum hose but on this engine it is under pressure?) is just the pressure reference for the regulator from the air side. There is a spring inside that is tuned to maintain the fuel pressure at 10psi higher than the air pressure, as you mentioned. A diaphragm separates the air and fuel inside the regulator, so the hose coming from the fuel regulator should only have air in it, right? I mentioned previously that I had a problem with this hose splitting and stalling the engine. I found the split because I could see fuel spraying out of the split. Not a lot, but enough for it to splatter on the air rail and to indicate that there is some fuel in this small hose. I assume that this means the diaphragm in the fuel regulator is broken, allowing fuel through (and assuming that the fuel does not make its way into this hose all the way from the fuel injectors). I also found by squirting soapy water on the clamps of this hose that I am still getting some leakage, so I need to order better clips.

I won't be able to work on or test the engine now for some months but looks like I need to clamp up the hose tightly and replace the fuel regulator. I have read that a classic symptom of a faulty fuel regulator is sooty plugs.

Thanks again for all your help.

Hi Paul,

Any feedback on this? Does the evidence point towards a broken fuel regulator? I'm not sure if I would expect to see a lot of fuel pouring out of the fuel regulator hose as opposed to some spraying out. The cut off section of hose smells a lot like petrol so I assume the spraying liquid was not something like condensed water vapour in the air.

Thanks again for all your help.